1. Field of the Invention
The present invention relates to an optical modulator used for an optical communication, and in particular, to a Mach-Zehnder optical modulator using an optical waveguide.
2. Description of the Related Art
For example, as optical waveguide devices using an electro-optic crystal such as lithium niobate (LiNbO3), lithium tantalate (LiTaO2) or the like, a variety of functional devices is formed such that a metallic film is formed on a part of a crystal substrate, to be thermally diffused or to be patterned, then, is proton exchanged in benzoic acid so that an optical waveguide is formed, and thereafter, an electrode is disposed in the vicinity of the optical waveguide. As one of optical waveguide devices using the electro-optical crystals, there has been known a Mach-Zehnder optical modulator having an optical waveguide structure of branching interference type.
FIG. 9 is a configuration diagram showing one example of conventional Mach-Zehnder optical modulators, in which (A) is a plan view and (B) is an X—X cross sectional view.
In FIG. 9, the conventional Mach-Zehnder optical modulator has an optical waveguide structure comprising an incident waveguide 111, a branching section 112, a parallel waveguides 113A and 113B, a multiplexing section 114 and an emission waveguide 115, each formed on a substrate 101, and is provided with a coplanar electrode comprising a signal electrode 121 and an earth electrode 122 disposed on the parallel waveguides 113A and 113B. In this coplanar electrode, in the case where a Z-cut crystal substrate 101 is used for example, in order to utilize a change in refractive index due to an electric field in a Z direction, the signal electrode 121 and the earth electrode 122 are arranged respectively just above the parallel waveguides 113A and 113B. To be specific, the respective electrodes 121 and 122 are patterned on the parallel waveguides 113A and 113B. However, in order to prevent lights being propagated through the parallel waveguides 113A and 113B from being absorbed by the signal electrode 121 and the earth electrode 122, a buffer layer 102 is formed between the crystal substrate 101, and the signal electrode 121 and the earth electrode 122. As the buffer layer 102, SiO2 of 0.2 to 1 μm thickness is used for example.
In the case where the conventional Mach-Zehnder optical modulator as described above is driven at a high speed, the signal electrode 121 is earthed at one end thereof via a resistor to be made a traveling wave electrode, and a high frequency electric signal S, such as a microwave or the like, is applied from the other end of the signal electrode 121. At this time, each refractive index of the parallel waveguides 113A and 113B is changed by +ΔnS and −ΔnG, respectively, due to an electric field E generated between the signal electrode 121 and the earth electrode 122. Therefore, a phase difference between the lights being propagated through the parallel waveguides 113A and 113B is changed, so that a signal light intensity modulated is output from the emission waveguide 114.
Further, it is also possible to obtain an optical response characteristic of broadband, by changing a cross section of the electrode to control the effective refractive index of the microwave, and by matching propagation speeds of the light and the microwave with each other. Moreover, as shown in FIG. 10 for example, there has been known an optical modulator of a configuration in which the configuration of FIG. 9 is connected serially in two stages. In such an optical modulator, a clock signal is given to the signal electrode 121 on the former stage and a data signal is given to the signal electrode 121′ on the latter stage, so that a modulated light of RZ (return to zero) format or the like can be generated.
However, the conventional Mach-Zehnder optical modulator as described above has a following problem related to the wavelength chirping. Namely, in the conventional Mach-Zehnder optical modulator, the intensity of the electric field E applied on the respective parallel waveguides 113A and 113B is varied depending on the arrangements of the signal electrode 121. Therefore, a change amount (ΔnS) in the refractive index of the parallel waveguide 113B near the signal electrode 121 becomes larger than a change amount (ΔnG) in the refractive index of the parallel waveguide 113A far from the signal electrode 121. As a result, there is caused a problem in that absolute values of phase changes in the respective lights being propagated through the parallel waveguides 113A and 113B are varied, and when a signal is switched from “0” to “1” or from “1” to “0”, a wavelength change (wavelength chirping) in the modulated light is caused to degrade a signal waveform after transmitted.
In order to reduce the wavelength chirping, there are a method of using an X-cut crystal substrate, a method of arranging two signal electrodes respectively on the parallel waveguides to push-pull drive the Mach-Zehnder optical modulator, and the like.
In the case where the X-cut crystal substrate is used, it becomes possible to perform the modulation in which the wavelength chirping is not caused, by applying electric fields of +z direction and −z direction respectively on two parallel waveguides. However, since it is impossible to arrange the parallel waveguides just below the signal electrode, there is a drawback in that a distance between the signal electrode and the waveguide is lengthened, and accordingly, a high drive voltage needs to be applied.
Further, in the case where the two signal electrodes are used to perform the push-pull driving, since two connectors for inputting the high frequency electric signal are necessary, and also electric signals whose data is inverted to each other need to be applied on both of the signal electrodes while phases thereof being controlled, there is a drawback in that a circuit configuration of driving system becomes complicated.
The present invention has been accomplished in view of the above problems, and has a first object to provide a Mach-Zehnder optical modulator capable of reducing the wavelength chirping caused in a modulated light. Further, a second object of the present invention is to provide a Mach-Zehnder optical modulator capable of outputting a modulated light in which the desired wavelength chirping is caused.
Note, Japanese Unexamined Patent Publication No. 53-90747 discloses a polarization inversion optical modulator. This polarization inversion optical modulator is configured such that a region whose polarization direction is inverted is disposed in a strip shape within a two-dimensional waveguide and a voltage is applied between two electrodes which are disposed so as to put in parallel the polarization inversion region therebetween, so that a light being propagated through the polarization inversion region is modulated. Such a polarization inversion optical modulator can be used irrespective of single mode or multi mode, and has advantages in that a polarizer or an analyzer is not necessary, the dependence thereof on the temperature is low, and the like. However, the polarization inversion optical modulator described above has basically an optical waveguide structure different from that of the Mach-Zehnder optical modulator, and therefore, does not solve the problems of wavelength chirping as described above. Further, the object of the polarization inversion optical modulator is different from the object of the present invention.